The present invention is concerned with a process and a device for continuously anodically oxidizing one surface of a strip-shaped aluminum or aluminum alloy, which can particularly be used as a support material for offset-printing plates.
Strip-shaped aluminum which has been roughened and anodically oxidized is, for example, required for manufacturing electrolytic capacitors, in the building industry, for packaging materials, or in the production of support materials for offset-printing plates. For these purposes, the strip material is generally cut into smaller sizes.
Support materials for offset-printing plates are provided, on one or both sides, with a radiation-sensitive (photosensitive) coating (reproduction coating), which is applied either directly by the user or by the manufacturer of precoated printing plates and with the aid of which a printing image of an original is produced by a photomechanical route. Following the production of this printing form from the printing plate, the coating support comprises image areas which are ink-receptive in the subsequent printing process while, simultaneously with the image-production, a hydrophilic image-background for the lithographic printing operation is formed in the areas which are free from an image (non-image areas) in the subesequent printing process.
A coating support for reproduction coatings used in the manufacture of offset-printing plates must meet the following requirements:
Those portions of the radiation-sensitive coating, which are comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue and without any stronger attack on the support material by the developer.
The support, which has been laid bare in the nonimage areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink.
The radiation-sensitive coating must exhibit an adequate degree of adhesion prior to exposure, and those portions of the coating which print must exhibit adequate adhesion following exposure.
The support material should possess good mechanical stability, for example with respect to abrasion, and good chemical resistance, especially with respect to alkaline media.
As the base material for coating supports of this kind, aluminum is particularly frequently used, the surface of this aluminum being roughened, according to known methods, by dry-brushing, slurry-brushing, sandblasting, or by chemical and/or electrochemical treatments. In order to increase the resistance to abrasion, especially electrochemically roughened substrates are additionally subjected to an anodizing step, in order to build up a thin oxide layer. These anodic oxidation processes are conventionally carried out in aqueous electrolytes which contain H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.2 C.sub.2 O.sub.4, H.sub.3 BO.sub.3, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof. The oxide layers built up in these aqueous electrolytes or electrolyte mixtures differ from one another in structure, layer thickness and resistance to chemicals. As already mentioned above, roughened and anodically oxidized materials of this type are of some importance also in other technical fields. In the commercial production of supports for offset-printing plates, aqueous solutions of H.sub.2 SO.sub.4 and/or H.sub.3 PO.sub.4 are, in particular, used.
The prior art has disclosed the following devices and/or processes for continuously anodically oxidizing aluminum strips in a technically appropriate procedure. These devices and/or processes can fundamentally be divided into two groups:
1. The aluminum strip is made the anode by means of a contact roll (contact roller or contact bar) which is positioned outside the anodizing electrolyte and is connected to the positive pole of a d.c. supply. At least one cathode is arranged in the electrolyte and the aluminum strip is anodically oxidized on its surface which faces this electrode (see also FIG. 3 of the accompanying drawing).
2. The aluminum strip is made the anode by means of a contacting cell (contacting compartment) which is filled with an electrolyte and includes at least one anode. The strip itself is then passed as a center conductor into a second cell (compartment) filled with an electrolyte, in which at least one cathode is arranged (see also FIG. 4 of the accompanying drawing). In various variations of this arrangement, the sequence of cells (compartments) can be changed and it is also possible to use different electrolytes. The aluminum strip is anodically oxidized on that surface which faces the cathode.
The two variants 1 and 2 are, for example, described in DE-A No. 1,621,115 (=U.S. Pat. Nos. 3,632,468 and 3,766,043). Variant 1 or modifications of this variant are also disclosed in the following publications: DE-B No. 1,298,823 (=U.S. Pat. No. 3,296,114) including contacting electrode blocks outside the compartment which is filled with an electrolyte; DE-B No. 1,906,538 (=GB-A No. 1,260,505) having a contact-making brush outside the anodizing chamber; DE-C No. 2,045,787 (=U.S. Pat. No. 3,692,640) comprising a contacting electrolyte stream which is ejected from a hollow cathode device provided with openings, however, this publication does not clearly specify the kind of anodic connection; DE-C No. 2,234,424 (=U.S. Pat. No. 3,871,982) or DE-A No. 2,619,821, including, in each case, a contact roll outside the anodizing chamber.
Variant 2 or modifications of this variant are also disclosed in the following publications: DE-B No. 1,496,714 (=U.S. Pat. Nos. 3,471,375 and 3,359,189) comprising anode(s) disposed in a contacting and cleaning compartment filled with an electrolyte, which is arranged upstream of the anodizing compartment equipped with cathode(s), which is likewise filled with an electrolyte and in which the electrolyte flow is counter to the direction of travel of the aluminum strip; DE-A No. 2,156,677 (=U.S. Pat. No. 3,718,547) comprising a similar arrangement which additionally includes a downstream electrolyte-filled contacting compartment equipped with anode(s); DE-A No. 2,420,704 (=U.S. Pat. No. 3,865,700), in which the sequence of cells is reversed, i.e. the anodizing cell provided with a cathode is followed by the contacting cell provided with an anode; DE-B No. 2,507,063 (=U.S. Pat. No. 4,226,680) or DE-C No. 2,534,028 (=U.S. Pat. No. 3,959,090) including an anodic-oxidation stage and a coloring stage, in which the first stage also comprises a contacting compartment with anode(s) and an anodizing compartment with cathode(s); DE-A No. 2,853,609 (=GB-A No. 2,012,305) including anode(s) in a contacting compartment and cathode(s) in an anodizing compartment, in which the contacts of the cathodes show a specific predetermined arrangement; EP-B No. 0,007,233 comprising an anode in a contacting cell filled with an aqueous solution of H.sub.3 PO.sub.4 and a cathode in an anodizing cell filled with an aqueous solution of H.sub.2 SO.sub.4.
Variant 1 has the following disadvantages: The aluminum strip must be as dry as possible when it makes contact with the contact roll--notwithstanding preceding treatment steps in solutions, which are normally carried out--and, therefore, additional costs of construction and energy are required for an intermediate drying process. In addition, arc discharges may occur, when the strip is separated from the contact roll and these arc discharges can irreversibly destroy the surface of the aluminum strip and give rise to faults in the subsequent anodic oxidation or even render the strip entirely useless. In view of the high operating speeds currently demanded, for example, of 300 m/min and even higher, in connection with the high current densities required in the process, these disadvantages can prove to be particularly detrimental. These disadvantages do not occur with variant 2, however, in this variant, use of a contacting cell or contacting compartment, respectively, results in an additional extension of the anodic-oxidation stage to almost twice its length, and this is very uneconomical at the required operating speeds which necessarily involve long electrolyte baths.
DE-A No. 2,917,383 (=U.S. Pat. No. 4,214,961) describes a process for continuously electrochemically treating (roughening or anodically oxidizing) aluminum strips, in a vertical arrangement. In the process, the aluminum strip is, in each case, passed vertically over idler rollers and between dividers arranged in an electrolytic bath, with at least part of these dividers being also electrodes. According to FIG. 4 of this publication, all the dividers may be connected to act as electrodes; if this is so, two adjacent dividers are, in each case, anodes or cathodes, respectively, and, as a result, the aluminum is treated on both surfaces. In this variant, the process cannot be used for treating aluminum on one surface and it also cannot be applied to a strip which is substantially horizontally guided.